In recent years, SAW (surface acoustic wave) filters and BAW (bulk acoustic wave) filters have been used widely for RF filters for mobile communications such as for mobile phones. BAW filters include piezoelectric thin film resonators. There are two types of piezoelectric thin film resonators: FBAR (Film Bulk Acoustic Resonator) and SMR (Solidly Mounted Resonator). An FBAR has a structure in which an upper electrode, a piezoelectric film and a lower electrode are provided on a substrate as main components, and an air gap is formed under the lower electrode at a portion where the upper electrode and the lower electrode oppose each other. Here, the air gap is formed by wet etching a sacrificial layer provided on the surface of or inside the substrate, or wet etching or dry etching the substrate from the backside. On the other hand, an SMR has a structure in which, instead of the air gap, a laminate with a film thickness of λ/4 (λ: wavelength of acoustic wave) formed by alternately laminating films having a high acoustic impedance and films having a low acoustic impedance is provided, and the laminate is utilized as an acoustic reflection film.
In particular, filters and branching filters using BAWs have been receiving attention due to their higher Q-value even at high frequencies and smaller losses than those using conventional SAWs. However, as demands for lower power consumption in the field of mobile communications have become vigorous in recent years, the filters and the branching filters using BAWs are demanded for a further reduction in losses. Because of these reasons, low-loss piezoelectric thin film resonators have been actively developed.
One of the factors that contributes to losses in such filters using BAWs is that acoustic waves leak to the outside (hereinafter referred to as a non-resonance portion) of an area where the upper electrode and the lower electrode oppose each other (hereinafter referred to as a resonance portion), in other words, acoustic waves leak into an area where they are less likely to be reconverted into electric signals, and thereby causing losses. Herein, this phenomenon will be referred to as a “lateral leakage”. The lateral leakage is caused by the magnitude relationship in acoustic velocity between the resonance portion and the non-resonance portion. The magnitude relationship in acoustic velocity that suppresses the lateral leakage is determined by the Poisson's ratio of a piezoelectric material to be used. The acoustic velocity in the resonance portion becomes lower than that in the non-resonance portion when the Poisson's ratio is 1/3 or more, and the acoustic velocity in the resonance portion becomes higher than that in the non-resonance portion when the Poisson's ratio is 1/3 or less.
Here, in a case where a piezoelectric film is formed using a material whose Poisson's ratio is 1/3 or more, the acoustic velocity in the resonance portion becomes lower than that in the periphery when an appropriate amount of mass is added to the resonance portion. Thus, the lateral leakage can be suppressed with relative ease. In contrast, in a case where a piezoelectric film is formed using a material whose Poisson's ratio is 1/3 or less, the acoustic velocity relationship that suppresses the lateral leakage becomes opposite. Thus, it is difficult to suppress the lateral leakage. In currently-practical filters using piezoelectric thin film resonators, AlN, whose Poisson's ratio is 1/3 or less, is used for the piezoelectric films. Thus, it is difficult to suppress the lateral leakage, and as a result the losses increase.
As a way to solve the lateral leakage of acoustic waves, Japanese Laid-open Patent Application No. 2007-300430 discloses a resonator in which the piezoelectric film in the resonance portion is subjected to patterning and at least a portion of the periphery of the patterned piezoelectric film is provided inwardly than an area where the upper electrode and the lower electrode oppose each other. By using the resonator disclosed in Japanese Laid-open Patent Application No. 2007-300430, the lateral leakage of acoustic waves can be suppressed in a highly effective manner.
The Q-value of the resonator disclosed in the patent document can be increased by increasing the over-etching amount of the piezoelectric film to further increase the length of a hood-like end portion. When the length of the hood-like end portion is increased, the mechanical strength of the end portion is difficult to maintain. Accordingly, with the resonator disclosed in the patent document, it is difficult to increase the Q-value while maintaining the mechanical strength of the end portion.